Disease outlook for the 2020 crop season

We can’t be sure which diseases will attack this year, but indications are that once they arrive, they’ll be here to stay

What will be the big disease concern this year? Will it be tar spot in corn, or frog-eye leaf spot in soybeans? Or will growers be facing more of the same, with soybean cyst nematode and northern corn leaf blight?

Forecasting diseases is iffy at best. The truth is that most disease pathogens, once they establish in Ontario or Eastern Canada, have the potential to affect crop production.

For 2020, the message on disease management is simply “be prepared.” Diseases require specific conditions in order to flourish and have an impact on production. “The Disease Triangle” dictates that if a pathogen lands on a host plant under the right environmental conditions, a disease will flourish.

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Remove one of those parameters, it cannot develop.

Disease dynamics aside, there is a concern that growers in Eastern Canada are paying too much attention to what’s on the horizon without keeping track of the near-and-present diseases that are here already. Tar spot in corn appeared on the radar in 2019, serving as a distraction from the problems with gibberella in 2018. But the fact is, tar spot is not in Ontario and even when — not if — it appears, chances are the initial infection will be sporadic and less severe.

“There is increased attention to some of the new problems like tar spot in corn but my concern is around those diseases that overwinter on crop residues,” says Drew Thompson, market development agronomist with Pride Seeds. “Although disease pressure was low for most of Ontario in 2019 in both corn and soybeans, at the tail end of the growing season, it became much easier to find infections.”

Thompson cites field conditions that challenged growers who used tillage as a means of controlling or burying pathogens at the end of 2019. As a result, those with either corn-on-corn or soybeans-on-soybeans could be more likely to see elevated disease pressure from either northern corn leaf blight or frogeye leaf spot in soybeans in 2020.

Nor are those the only two diseases worth noting, and Thompson maintains that the best approach is to be prepared for the unexpected and look for diseases that are entrenched and pose the greatest challenge. Those include soybean cyst nematode (SCN), sudden death syndrome (SDS) and white mould in soybeans. On the corn side, northern corn leaf blight and gibberella (GER) are two consistent threats.

Soybean cyst nematode continues to rob yields and spread further abroad across Ontario.
photo: Courtesy of Albert Tenuta (OMAFRA)

“Most diseases we face are ‘native,’ in that they’re diseases that have been around since the crops they infect were first grown,” says Thompson. “As such, they’re well-suited to our climate, which includes winter.”

On the alert

In dealing with the potential for infections, the best tool — beyond crop plans or pencilling fungicide costs — is to be prepared, as Thompson advises. But it’s not just a matter of a grower knowing they have SCN in a field, it’s understanding which diseases are more prevalent in a crop and how they have an impact on plant health and ultimately yield.

Soybean cyst nematode is especially troubling to Peter Johnson. It’s not just that SCN is one of the greatest disease threats to soybeans, it’s that it goes undetected in so many fields. He speaks of the 2015 survey of parts of midwestern Ontario, where higher-than-expected concentrations were found in Huron and Bruce counties, at levels that could only have developed over many years, not as a recent find.

“Everyone needs to be more aware of where SCN is and that it’s a wider-spread problem than a lot of growers think,” says Johnson, an independent agronomist. “There are more farmers who are growing more soybeans because they tend to be profitable and you’re not working with wet soil. But whether it’s corn-soybean-wheat-soybean or soybean-corn-soybean-wheat, it means 50 per cent soybeans, and too many growers grow too many soybeans. The more soybeans in the rotation, the more of an issue SCN is and the more of an issue SDS will become, and the poorer soil quality you’ll have.”

Of the so-called “Big Three,” soybeans are the hardest crop on the soil, adds Johnson. Their shallow roots do little to improve structure and they’re large users of phosphorus and potash, returning little to the soil. Yet in spite of soybeans’ rise in popularity, the lack of awareness of the diseases that are the biggest threat stands in stark contrast to wheat, which Johnson notes is every bit as challenging to manage as soybeans, from a disease perspective. Fusarium head blight is a constant threat in wheat production and has the potential to affect the crop in any year.

“Growers know that when you grow wheat, you manage for fusarium — it’s just accepted,” says Johnson, noting that gibberella ear rot in corn presents a different threat, since growers might see GER only one year in 10 or one in eight. “Some of the more sporadic diseases can lull you into a false sense of security. You should always be on the lookout for the other diseases.”

That’s why Johnson is adamant that growers need to be aware of which diseases matter to their crops, and pay attention to local alerts and media reports of their advance.

Improved varietal screening and greater awareness on fungicide use and timing has lessened the impact of fusarium head blight in wheat.
photo: Courtesy of Albert Tenuta (OMAFRA)

“Pay attention to Twitter, pay attention to whatever source — whether it’s your local co-op or retailer,” says Johnson, adding that he provides regular updates on his podcast. “Pay attention to a resource where you’re going to get that alert that tells you to get out to the field and look.”

Network in place

In terms of staying informed, Albert Tenuta notes researchers, extension personnel and agronomists are sharing information and that this has created a clearer picture of the diseases having an impact on crops in Ontario. He and his colleagues are communicating through the Crop Protection Network with the goal of anticipating how losses to various diseases are having an impact on production, including in Ontario.

“Whether it’s corn, soybeans or wheat, we have a network of colleagues throughout North America,” says Tenuta, field crops pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). “We communicate on a regular basis to discuss where we’re at, where we’re going to be and what the future may hold.”

He cites Goss’s wilt and tar spot as good examples of corn diseases that are well known and have been in the U.S. for years. Both are confirmed in Michigan yet neither has entered Ontario.

“We do a survey every year, specifically for diseases that are here, and the ones that are coming are important to track, but when they do arrive, the incidence and severity that first year will be minimal,” says Tenuta. He refers to the annual corn disease survey which includes tar spot, conducted with Agriculture and Agri-Food Canada (AAFC) and Grain Farmers of Canada (GFO) with CAP funding. In 2019, the disease was never found. “It takes time to build up and establish, but the moment tar spot appears, everyone in the province is going to know.”

It’s important for growers to monitor the diseases that are in their fields and those that can cause problems every year and to keep good records and plan for in-season responses. For instance, with SCN and SDS, it’s vital to know that resistant varieties are the best way of managing these diseases. In white mould, row-widths and planting dates can affect the potential for the disease to develop, where narrower rows lead to quicker canopy closure and the development of the right environment for mould to develop.

Do the math

For some, applying fungicides is seen as cheap insurance, but Thompson stresses more thought must be put into the decision. Under certain circumstances, margins can be tight, which can influence a grower to plant one crop versus another.

“When working on cost of production, pencil in a fungicide application and if you end up in the red, then you likely should look at another crop,” Thompson says. “We have forecasts and other predictive means to try to guess if a fungicide will be necessary, but there are years when despite all the signs saying a fungicide is needed, there is minimal disease pressure and minimal yield response.”

Parts of Ontario see white mould in soybeans as a near-perennial issue.
photo: Courtesy of Albert Tenuta (OMAFRA)

Against that outlook, Thompson says fungicide use in a crop like corn has to be judicious. He knows from his past work in research that cereals are the most responsive to fungicides and the most likely to show a positive return on investment.

“Research on fungicide applications in corn is starting to show promise, particularly when fungicide applications are made in combination with other management decisions,” says Thompson.

As examples, he cites higher plant populations and higher fertility programs in corn. In soybeans, some growers are starting to work with two-pass applications of fungicides.

“A single fungicide application may not elicit a positive return but when done in combination, the response can be impressive,” he says.

Still, fungicide use in corn or soybeans has to be justified.

More IPM?

Integrated Pest Management (IPM) is often held up as a silver bullet for managing diseases as well as insect pests and weeds. Yet Johnson and Tenuta state that in many ways, IPM is already at the heart of growers’ efforts to manage diseases, insects and weeds.

From Tenuta’s vantage point, the contention that field crop producers aren’t practising IPM couldn’t be further from the truth. Farmers are planting resistant varieties, they’re walking and scouting their fields, and they’re targeting fungicides appropriately.

They also look to stay on top of emerging developments.“We have the information network amongst the pathologists, consultants and OMAFRA personnel, so use the expertise of the people scouting and monitoring the situation, not just here but elsewhere,” says Tenuta. “Use that to guide you to get out there.”

For Johnson, the key to IPM is a three-word phrase he’s been uttering since the late 1980s.

“The absolute best way to employ integrated pest management is ‘rotation, rotation, rotation,’” he says. “The more crops we can get into rotation, the better. But a two-crop system, whether it’s corn-soybeans or soybeans-wheat, that is not a rotation, that is an alternation and with alternations, insects and diseases can adapt.”

A true rotation — with a minimum of three crops is the best integrated pest management strategy, according to Johnson. Add two years of hay and that will further disrupt diseases and pests.

Use of trait technologies, rotating modes of action and even intermittent tillage may be an answer. Often, the simplest strategies are the best.

About the author

CG Production Editor

Ralph Pearce

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